Heretofore, an electrostatic latent image formed in an electrophotography has been mainly developed in a dry type development process. The dry type development process is divided into two types; namely, one in which a single-component type developer mainly comprising magnetic tone is used, and the other in which two-component type developer comprising non-magnetic toner and magnetic carrier.
The latter type developer is preferably used from such a viewpoint that a desired electric charge can be provided by mechanically stirring toner and carrier. Such a magnetic carrier as mentioned above is required to have a proper frictional chargeability, a fuidity, a developability and a high durability in a long-term use and, besides, it is so desired as not to damage an image-forming member surface in the courses of carrying out a contact-development, a cleaning treatment and so forth.
Accordingly, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP OPI Publication) Nos. 60-66264/1985 and 60-66265/1985, for example, propose respectively the resin-coated carriers each prepared by coating and then drying a solution containing a dissolved resin on the surface of a core-member comprising a ferromagnetic metal or the oxide thereof. The above-mentioned carriers have a high resistance. Therefore, the carriers can prevent an electrostatic latent image disorder produced by an outflow of a charge from the latent image, and the carriers can also prevent a carrier adhesion to an image-forming member when a charge is applied from a development sleeve. Further, the carriers have many advantages including, for example, such an advantage that the surface of an image-forming member cannot be damaged in the courses of carrying out a contact-development and a cleaning treatment.
However, the photographic characteristics of a carrier, such as a frictional chargeability, fluidity, developability and durability, are greatly influenced by the kinds of resins capable of forming a coated layer and, besides, the processes for forming a coated layer. Accordingly, JP OPI Publication No. 59-104664/1984 proposes a technique for forming a resin-coated carrier, in which the core-member surface of a carrier is coated with an independent polymer prepared, in a wet process such as a dipping, spraying or fluidized-bedding process, by polymerizing a monomer of cycloalkyl methacrylate that is an aliphatic methacrylate, or a copolymer of the above-mentioned monomer and a monomer of styrene, vinyl acetate, vinyl chloride or the like. The same JP OPI Publication also describes that a carrier excellent in frictional chargeability and fluidity and, particularly in moisture resistance can be prepared.
The above-mentioned resin-coated carrier described in JP OPI Publication No. 59-104664/1984 has such an advantage that the moisture resistance is excellent and that the outflow of a charge from a latent image can be reduced. On the other hand, however, there is such a problem that the adhesive property to the core-member of a resin-coated layer is poor, and that the core-member is bared by peeling a coated layer out of the core-member in a long-term use. Therefore, toner is spent on the bared core-member so as to deteriorate a chargeability in a long-term use.
According to the studies made by the inventors, when using the above-mentioned aliphatic methacrylate independently, it is liable to serious abrasion, due to the stress given by a developing unit or the like. Thereby, the charged level is suddenly dropped from the point of time when the coated layer has a certain thickness. Therefore, it is not suitable for a long-life developer.
On the other hand, JP OPI Publication No. 63-37360/1988 proposes a technique for forming a resin-coated layer, in which polymethyl methacrylate prepared by polymerizing a methyl methacrylate monomer, i.e., a chain-type methacrylate monomer that is served as the fine particles of resin, and the fine particles of resin are fixed to the surface of a carrier core member by applying a mechanical impact that is so-called a dry process. Also, JP OPI Publication No. 63-235965/1988 proposes a technique for forming a resin-coated layer, in which polystyrene-methyl methacrylate prepared by copolymerizing a chain-type methacrylate monomer, i.e., a methyl methacrylate monomer, and a styrene monomer, that is served as the fine particles of resin, and the fine particles of resin are fixed to a carrier core member by applying a mechanical impact, in the same dry process. The above-mentioned JP OPI Publication No. 63-235965/1988 describes that the resin of the resulting coated layer is excellent in adhesive property to the core member, that the coated layer is formed in a dry process by fixing the fine particles of the resin with a mechanical impact to the core member and, therefore, that a highly durable carrier can be provided without producing any delamination of the coated layer even in long-term use.
However, the above-mentioned chain-type methacrylate polymer coated-layer is hydroscopic in the material of itself. Therefore, when making use of a developer containing a carrier provided with the above-mentioned coated-layer under the highly humid conditions after preserving it for a long time, or when an image formation is carried out for a long time under the highly humid conditions, there may raise such a problem that an image quality is deteriorated by producing the leakage of charge, that a fog is produced on the background, or that a carrier adhesion is produced. The adhesive force of a toner to a carrier surface is high under the highly humid conditions (due to the influence of a moisture adsorption). Therefore, there may also raise such a problem that a toner is liable to be spent on a carrier surface.
According to the studies made by the inventors, when the above-mentioned chain-type methacrylate polymer is used independently, it is allowed to say that a coated-layer cannot be abraded at all. Therefore, there may raise such a problem that the chain-type methacrylate polymer has a limited function to remove a spent toner, and that the charged amount thereof is gradually lowered.